Engineered T cell therapies have been remarkably successful in the treatment of B cell malignancies, yet lack of control over these “living drugs” can lead to significant toxicities or limited efficacy. One particular challenge is the achievement of durable anti-tumor responses because the reduction of tumor burden results in reduced antigen stimulation and therefore reduced antigen-dependent T cell expansion. Interleukin-15 (IL15) drives T and NK cell expansion and persistence in an antigen-independent manner, however unregulated expression of this cytokine may compromise the safety and efficacy of cellular immunotherapy. To address these issues, we engineered T cells with a pharmacologically controllable, membrane-bound IL15 that supports antigen-independent T cell expansion and has the potential to reduce the safety risks associated with continuous exposure to soluble IL15. Our approach utilizes Drug Responsive Domains (DRDs) which are fully human protein domains that are inherently unstable in the cell but are reversibly stabilized when bound to specific FDA-approved drugs. Fusion of a DRD to a protein of interest confers drug-dependent, reversible regulation of protein expression and function. We developed a DRD based on the carbonic anhydrase 2 (CA2) protein, which is stabilized in the presence of the FDA-approved drug acetazolamide (ACZ). Upon gene transfer of membrane-bound IL15 fused to a CA2 DRD, regulated IL15 expression on T cells was detected only in the presence of ACZ. In the absence of ACZ, the level of IL15 detected on the surface of gene-modified T cells is not substantially different from the level detected on untransduced T cells. ACZ treatment of gene-modified T cells increases surface IL15 expression in a dose-dependent manner. We observed prolonged survival and up to 15-fold expansion of IL15-CA2 DRD-transduced T cells in the absence of supplemental cytokines or antigen stimulation. In contrast, vehicle-treated IL15-CA2 DRD modified T cells and untransduced T cells did not survive or expand in vitro. Importantly, both IL15-CA2-transduced T cells and unengineered, co-infused NK cells survived and persisted significantly more in vivo in ACZ-treated but not vehicle-treated non-tumor-bearing NSG mice. Our results demonstrate that a novel membrane-bound IL15-CA2 fusion protein coupled with ACZ treatment induces antigen-independent T cell expansion and augments bystander NK cell persistence. Regulatable IL15 expression has significant implications for both T and NK cell therapies by providing more durable cell expansion and cytolytic activity in vivo, and thus the potential to significantly reduce cell dosing while maintaining clinical efficacy in patients.

Citation Format: Steven Shamah, Kutlu Elpek, Tucker Ezell, Michelle Fleury, Michael Gallo, Jennifer Gori, Scott Heller, Mara Inniss, Meghan Langley, Grace Olinger, Celeste Richardson, Karen Tran, Dhruv Sethi, Dexue Sun, Vipin Suri. Drug responsive domain regulation of IL15-engineered T cells provides pharmacological control over antigen-independent cell expansion [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6604.